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dhclient.conf(5)					      dhclient.conf(5)

       dhclient.conf - DHCP client configuration file

       The dhclient.conf file contains configuration information for dhclient,
       the Internet Software Consortium	DHCP Client.

       The dhclient.conf file is a free-form ASCII text	file.	It  is	parsed
       by  the	recursive-descent  parser  built into dhclient.	  The file may
       contain extra tabs and newlines for formatting purposes.	  Keywords  in
       the file	are case-insensitive.	Comments may be	placed anywhere	within
       the file	(except	within quotes).	  Comments begin with the #  character
       and end at the end of the line.

       The  dhclient.conf  file	 can be	used to	configure the behaviour	of the
       client  in  a  wide  variety  of	 ways:	protocol  timing,  information
       requested from the server, information required of the server, defaults
       to use if the server does not provide certain information, values  with
       which  to  override  information	 provided  by the server, or values to
       prepend or append to information	provided by the	server.	 The  configu-
       ration  file  can  also be preinitialized with addresses	to use on net-
       works that don't	have DHCP servers.

       The timing behaviour of the client need not be configured by the	 user.
       If no timing configuration is provided by the user, a fairly reasonable
       timing behaviour	will be	used by	default	- one which results in	fairly
       timely updates without placing an inordinate load on the	server.

       The  following statements can be	used to	adjust the timing behaviour of
       the DHCP	client if required, however:

       The timeout statement

       timeout time ;

       The timeout statement determines	the amount  of	time  that  must  pass
       between the time	that the client	begins to try to determine its address
       and the time that it decides that it's not going	to be able to  contact
       a  server.    By	 default,  this	 timeout is sixty seconds.   After the
       timeout has passed, if there are	any static leases defined in the  con-
       figuration  file,  or  any  leases remaining in the lease database that
       have not	yet  expired,  the  client  will  loop	through	 these	leases
       attempting  to  validate	 them,	and if it finds	one that appears to be
       valid, it will use that lease's address.	  If there are no valid	static
       leases  or  unexpired  leases  in  the  lease database, the client will
       restart the protocol after the defined retry interval.

       The retry statement

	retry time;

       The retry statement determines the time that must pass after the	client
       has  determined	that  there  is	no DHCP	server present before it tries
       again to	contact	a DHCP server.	 By default, this is five minutes.

       The select-timeout statement

	select-timeout time;

       It is possible (some might say desirable) for there to be more than one
       DHCP  server  serving any given network.	  In this case,	it is possible
       that a client may be sent more than one offer in	response to  its  ini-
       tial  lease  discovery message.	 It may	be that	one of these offers is
       preferable to the other (e.g., one  offer  may  have  the  address  the
       client previously used, and the other may not).

       The  select-timeout  is the time	after the client sends its first lease
       discovery request at which it stops waiting for	offers	from  servers,
       assuming	 that  it has received at least	one such offer.	  If no	offers
       have been received by the time  the  select-timeout  has	 expired,  the
       client will accept the first offer that arrives.

       By  default,  the  select-timeout is zero seconds - that	is, the	client
       will take the first offer it sees.

       The reboot statement

	reboot time;

       When the	client is restarted, it	first  tries  to  reacquire  the  last
       address	it  had.    This  is  called the INIT-REBOOT state.   If it is
       still attached to the same network it was attached to when it last ran,
       this  is	 the  quickest way to get started.   The reboot	statement sets
       the time	that must elapse after the client first	tries to reacquire its
       old  address  before  it	 gives up and tries to discover	a new address.
       By default, the reboot timeout is ten seconds.

       The backoff-cutoff statement

	backoff-cutoff time;

       The client uses an exponential backoff algorithm	with some  randomness,
       so  that	 if many clients try to	configure themselves at	the same time,
       they will not make their	requests  in  lockstep.	   The	backoff-cutoff
       statement  determines  the  maximum  amount  of time that the client is
       allowed to back off.   It defaults to two minutes.

       The initial-interval statement

	initial-interval time;

       The initial-interval statement sets the	amount	of  time  between  the
       first  attempt  to  reach  a  server  and the second attempt to reach a
       server.	Each time a message is sent, the interval between messages  is
       incremented by twice the	current	interval multiplied by a random	number
       between zero and	one.  If it is greater than the	backoff-cutoff amount,
       it is set to that amount.  It defaults to ten seconds.

       The  DHCP protocol allows the client to request that the	server send it
       specific	information, and not send it other information that it is  not
       prepared	 to  accept.	The  protocol also allows the client to	reject
       offers from servers if they don't contain information the client	needs,
       or if the information provided is not satisfactory.

       There  is  a variety of data contained in offers	that DHCP servers send
       to DHCP clients.	 The data that can be specifically requested  is  what
       are called DHCP Options.	 DHCP Options are defined in

       The request statement

	request	[ option ] [, ... option ];

       The  request  statement	causes	the  client to request that any	server
       responding to the client	send the client	its values for	the  specified
       options.	   Only	 the  option  names should be specified	in the request
       statement - not	option	parameters.    By  default,  the  DHCP	server
       requests	 the  subnet-mask,  broadcast-address,	time-offset,  routers,
       domain-name, domain-name-servers	and host-name options.

       In some cases, it may be	desirable to send no parameter request list at
       all.    To  do  this, simply write the request statement	but specify no


       The require statement

	require	[ option ] [, ... option ];

       The require statement lists options that	must be	sent in	order  for  an
       offer  to  be  accepted.	   Offers  that	 do not	contain	all the	listed
       options will be ignored.

       The send	statement

	send { [ option	declaration ] [, ... option declaration	]}

       The send	statement causes the client to send the	specified  options  to
       the  server  with the specified values.	These are full option declara-
       tions as	described in dhcp-options(5).  Options that are	always sent in
       the  DHCP protocol should not be	specified here,	except that the	client
       can specify  a  requested-lease-time  option  other  than  the  default
       requested  lease	 time,	which is two hours.  The other obvious use for
       this statement is to send information to	the server that	will allow  it
       to  differentiate  between  this	 client	 and other clients or kinds of

       The client now has some very limited support for	doing DNS updates when
       a  lease	 is  acquired.	 This is prototypical, and probably doesn't do
       what you	want.	It also	only works if you happen to have control  over
       your DNS	server,	which isn't very likely.

       To  make	 it  work,  you	 have to declare a key and zone	as in the DHCP
       server (see dhcpd.conf(5) for details).	 You also  need	 to  configure
       the fqdn	option on the client, as follows:

	 send fqdn.fqdn	"";
	 send fqdn.encoded on;
	 send fqdn.server-update off;

       The  fqdn.fqdn option MUST be a fully-qualified domain name.   You MUST
       define a	zone statement for the zone to be updated.   The  fqdn.encoded
       option  may  need  to be	set to on or off, depending on the DHCP	server
       you are using.

       The do-forward-updates statement

	do-forward-updates [ flag ] ;

       If you want to do DNS updates in	the DHCP client	script (see  dhclient-
       script(8))  rather  than	 having	the DHCP client	do the update directly
       (for example, if	you want to use	SIG(0) authentication,	which  is  not
       supported  directly by the DHCP client, you can instruct	the client not
       to do the update	using the do-forward-updates statement.	  Flag	should
       be  true	if you want the	DHCP client to do the update, and false	if you
       don't want the DHCP client to do	the update.    By  default,  the  DHCP
       client will do the DNS update.

       In  some	 cases,	a client may receive option data from the server which
       is not really appropriate for that client, or may not receive  informa-
       tion  that  it needs, and for which a useful default value exists.   It
       may also	receive	information which is useful, but  which	 needs	to  be
       supplemented  with  local information.	To handle these	needs, several
       option modifiers	are available.

       The default statement

	default	[ option declaration ] ;

       If for some option the client should use	 the  value  supplied  by  the
       server, but needs to use	some default value if no value was supplied by
       the server, these values	can be defined in the default statement.

       The supersede statement

	supersede [ option declaration ] ;

       If for some option the client should always  use	 a  locally-configured
       value  or  values rather	than whatever is supplied by the server, these
       values can be defined in	the supersede statement.

       The prepend statement

	prepend	[ option declaration ] ;

       If for some set of options the client should use	a  value  you  supply,
       and  then  use  the values supplied by the server, if any, these	values
       can be defined in the prepend statement.	  The  prepend	statement  can
       only  be	 used for options which	allow more than	one value to be	given.
       This restriction	is not enforced	- if you ignore	it, the	behaviour will
       be unpredictable.

       The append statement

	append [ option	declaration ] ;

       If  for some set	of options the client should first use the values sup-
       plied by	the server, if any, and	then use values	you supply, these val-
       ues  can	be defined in the append statement.   The append statement can
       only be used for	options	which allow more than one value	to  be	given.
       This restriction	is not enforced	- if you ignore	it, the	behaviour will
       be unpredictable.

       The lease declaration

	lease {	lease-declaration [ ...	lease-declaration ] }

       The DHCP	client may decide after	some period of time (see PROTOCOL TIM-
       ING)  that it is	not going to succeed in	contacting a server.   At that
       time, it	consults its own database of old leases	 and  tests  each  one
       that  has not yet timed out by pinging the listed router	for that lease
       to see if that lease could work.	  It is	possible to define one or more
       fixed  leases in	the client configuration file for networks where there
       is no DHCP or BOOTP service, so that the	client can still automatically
       configure its address.	This is	done with the lease statement.

       NOTE:  the  lease statement is also used	in the dhclient.leases file in
       order to	record leases that have	been received from DHCP	servers.  Some
       of  the	syntax	for  leases  as	 described below is only needed	in the
       dhclient.leases file.   Such syntax is documented  here	for  complete-

       A  lease	 statement  consists  of the lease keyword, followed by	a left
       curly brace, followed by	one or more lease declaration statements, fol-
       lowed  by  a  right curly brace.	  The following	lease declarations are


       The bootp statement is used to indicate that  the  lease	 was  acquired
       using  the  BOOTP protocol rather than the DHCP protocol.   It is never
       necessary to specify this  in  the  client  configuration  file.	   The
       client uses this	syntax in its lease database file.

	interface "string";

       The  interface  lease  statement	 is  used to indicate the interface on
       which the lease is valid.   If set, this	lease will only	be tried on  a
       particular interface.   When the	client receives	a lease	from a server,
       it always records the interface number on which it received that	lease.
       If  predefined  leases  are  specified  in  the dhclient.conf file, the
       interface should	also be	specified, although this is not	required.

	fixed-address ip-address;

       The fixed-address statement is used to set the ip address of a particu-
       lar  lease.    This  is	required  for  all  lease statements.	The IP
       address must be specified as a dotted quad (e.g.,

	filename "string";

       The filename statement specifies	the name of the	boot filename to  use.
       This  is	 not  used by the standard client configuration	script,	but is
       included	for completeness.

	server-name "string";

       The server-name statement specifies the name of the boot	server name to
       use.    This  is	 also  not  used  by the standard client configuration

	option option-declaration;

       The option statement is used to specify the value of an option supplied
       by  the	server,	 or,  in  the  case  of	 predefined leases declared in
       dhclient.conf, the value	that the user wishes the client	 configuration
       script to use if	the predefined lease is	used.

	script "script-name";

       The script statement is used to specify the pathname of the dhcp	client
       configuration script.  This script is used by the dhcp  client  to  set
       each  interface's initial configuration prior to	requesting an address,
       to test the address once	it has been offered, and  to  set  the	inter-
       face's  final  configuration  once  a  lease has	been acquired.	 If no
       lease is	acquired, the script is	used to	 test  predefined  leases,  if
       any,  and  also	called once if no valid	lease can be identified.   For
       more information, see dhclient-script(8).

	vendor option space "name";

       The vendor option space statement is used to specify which option space
       should  be  used	 for decoding the vendor-encapsulate-options option if
       one is received.	 The dhcp-vendor-identifier can	be used	to  request  a
       specific	class of vendor	options	from the server.   See dhcp-options(5)
       for details.

	medium "media setup";

       The medium statement can	be used	on systems  where  network  interfaces
       cannot  automatically  determine	 the type of network to	which they are
       connected.  The media setup  string  is	a  system-dependent  parameter
       which is	passed to the dhcp client configuration	script when initializ-
       ing the interface.  On Unix and	Unix-like  systems,  the  argument  is
       passed on the ifconfig command line when	configuring the	interface.

       The  dhcp  client  automatically	 declares  this	parameter if it	uses a
       media type (see the media statement) when configuring the interface  in
       order  to  obtain a lease.  This	statement should be used in predefined
       leases only if the network interface requires media type	configuration.

	renew date;

	rebind date;

	expire date;

       The  renew  statement  defines the time at which	the dhcp client	should
       begin trying to contact its server to renew a lease that	it  is	using.
       The  rebind  statement defines the time at which	the dhcp client	should
       begin to	try to contact any dhcp	server in order	to  renew  its	lease.
       The  expire  statement  defines	the time at which the dhcp client must
       stop using a lease if it	has not	been able to contact a server in order
       to renew	it.

       These declarations are automatically set	in leases acquired by the DHCP
       client, but must	also be	configured in predefined leases	- a predefined
       lease whose expiry time has passed will not be used by the DHCP client.

       Dates are specified as follows:

	_weekday_ _year_/_month_/_day_ _hour_:_minute_:_second_

       The weekday is present to make it easy for a human to tell when a lease
       expires	- it's specified as a number from zero to six, with zero being
       Sunday.	When declaring a predefined lease, it can always be  specified
       as  zero.   The year is specified with the century, so it should	gener-
       ally be four digits except for really long leases.  The month is	speci-
       fied  as	a number starting with 1 for January.  The day of the month is
       likewise	specified starting with	1.  The	hour is	a number between 0 and
       23,  the	minute a number	between	0 and 59, and the second also a	number
       between 0 and 59.

	alias {	 declarations ... }

       Some DHCP clients running TCP/IP	roaming	protocols may require that  in
       addition	 to  the lease they may	acquire	via DHCP, their	interface also
       be configured with a predefined IP alias	so that	they can have a	perma-
       nent  IP	address	even while roaming.   The Internet Software Consortium
       DHCP client doesn't support roaming with	fixed addresses	directly,  but
       in order	to facilitate such experimentation, the	dhcp client can	be set
       up to configure an IP alias using the alias declaration.

       The alias  declaration  resembles  a  lease  declaration,  except  that
       options	other  than the	subnet-mask option are ignored by the standard
       client configuration script, and	expiry times are ignored.   A  typical
       alias  declaration  includes  an	interface declaration, a fixed-address
       declaration for the IP alias address, and a subnet-mask option declara-
       tion.	A medium statement should never	be included in an alias	decla-

	reject ip-address;

       The reject statement causes the	DHCP  client  to  reject  offers  from
       servers	who  use  the specified	address	as a server identifier.	  This
       can be used to avoid being configured by	rogue  or  misconfigured  dhcp
       servers,	although it should be a	last resort - better to	track down the
       bad DHCP	server and fix it.

	interface "name" { declarations	...  }

       A client	with more than one network interface may require different be-
       haviour	depending on which interface is	being configured.   All	timing
       parameters and declarations other than lease and	alias declarations can
       be enclosed in an interface declaration,	and those parameters will then
       be used only  for  the  interface  that	matches	 the  specified	 name.
       Interfaces  for	which  there  is no interface declaration will use the
       parameters declared  outside  of	 any  interface	 declaration,  or  the
       default settings.

	pseudo "name" "real-name" { declarations ...  }

       Under some circumstances	it can be useful to declare a pseudo-interface
       and have	the DHCP client	acquire	a configuration	 for  that  interface.
       Each  interface	that the DHCP client is	supporting normally has	a DHCP
       client state machine running on it to acquire and maintain  its	lease.
       A  pseudo-interface is just another state machine running on the	inter-
       face named real-name, with its own lease	and its	own  state.    If  you
       use  this  feature,  you	 must provide a	client identifier for both the
       pseudo-interface	and the	actual interface, and the two identifiers must
       be  different.	You must also provide a	separate client	script for the
       pseudo-interface	to do what you want with the IP	address.    For	 exam-

	    interface "ep0" {
		 send dhcp-client-identifier "my-client-ep0";
	    pseudo "secondary" "ep0" {
		 send dhcp-client-identifier "my-client-ep0-secondary";
		 script	"/etc/dhclient-secondary";

       The  client  script  for	 the pseudo-interface should not configure the
       interface up or down - essentially, all it  needs  to  handle  are  the
       states where a lease has	been acquired or renewed, and the states where
       a lease has expired.   See dhclient-script(8) for more information.

	media "media setup" [ ,	"media setup", ... ];

       The media statement defines one or more media configuration  parameters
       which  may  be  tried  while attempting to acquire an IP	address.   The
       dhcp client will	cycle through each media setup	string	on  the	 list,
       configuring  the	interface using	that setup and attempting to boot, and
       then trying the next one.   This	can be	used  for  network  interfaces
       which  aren't  capable  of  sensing  the	media type unaided - whichever
       media type succeeds in getting a	request	to the server and hearing  the
       reply is	probably right (no guarantees).

       The  media setup	is only	used for the initial phase of address acquisi-
       tion (the DHCPDISCOVER and DHCPOFFER packets).	Once  an  address  has
       been acquired, the dhcp client will record it in	its lease database and
       will record the media type used to acquire the address.	 Whenever  the
       client  tries  to  renew	 the  lease, it	will use that same media type.
       The lease must expire before the	client will go back to cycling through
       media types.

       The  following  configuration  file  is used on a laptop	running	NetBSD
       1.3.   The laptop has an	IP alias of, and  has  one	inter-
       face,  ep0  (a  3com  3C589C).	 Booting intervals have	been shortened
       somewhat	from the default, because the client is	known to spend most of
       its  time on networks with little DHCP activity.	  The laptop does roam
       to multiple networks.

       timeout 60;
       retry 60;
       reboot 10;
       select-timeout 5;
       initial-interval	2;

       interface "ep0" {
	   send	host-name "";
	   send	dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
	   send	dhcp-lease-time	3600;
	   supersede domain-name "";
	   prepend domain-name-servers;
	   request subnet-mask,	broadcast-address, time-offset,	routers,
		domain-name, domain-name-servers, host-name;
	   require subnet-mask,	domain-name-servers;
	   script "/sbin/dhclient-script";
	   media "media	10baseT/UTP", "media 10base2/BNC";

       alias {
	 interface "ep0";
	 option	subnet-mask;
       This is a very complicated  dhclient.conf  file	-  in  general,	 yours
       should be much simpler.	 In many cases,	it's sufficient	to just	create
       an empty	dhclient.conf file - the defaults are usually fine.

       dhcp-options(5),	dhclient.leases(5), dhclient(8), RFC2132, RFC2131.

       dhclient(8) was written by Ted Lemon under a contract with Vixie	 Labs.
       Funding	for this project was provided by the Internet Software Consor-
       tium.  Information about	the Internet Software Consortium can be	 found



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